diff --git a/caps/nsJSPrincipals.cpp b/caps/nsJSPrincipals.cpp
index 61d95ebab7a6..aee99870c017 100644
--- a/caps/nsJSPrincipals.cpp
+++ b/caps/nsJSPrincipals.cpp
@@ -344,13 +344,14 @@ static bool WritePrincipalInfo(JSStructuredCloneWriter* aWriter,
          JS_WriteBytes(aWriter, aBaseDomain.get(), aBaseDomain.Length());
 }
 
-static bool WritePrincipalInfo(JSStructuredCloneWriter* aWriter,
-                               const PrincipalInfo& aInfo) {
+/* static */
+bool nsJSPrincipals::WritePrincipalInfo(JSStructuredCloneWriter* aWriter,
+                                        const PrincipalInfo& aInfo) {
   if (aInfo.type() == PrincipalInfo::TNullPrincipalInfo) {
     const NullPrincipalInfo& nullInfo = aInfo;
     return JS_WriteUint32Pair(aWriter, SCTAG_DOM_NULL_PRINCIPAL, 0) &&
-           WritePrincipalInfo(aWriter, nullInfo.attrs(), nullInfo.spec(),
-                              EmptyCString(), EmptyCString());
+           ::WritePrincipalInfo(aWriter, nullInfo.attrs(), nullInfo.spec(),
+                                EmptyCString(), EmptyCString());
   }
   if (aInfo.type() == PrincipalInfo::TSystemPrincipalInfo) {
     return JS_WriteUint32Pair(aWriter, SCTAG_DOM_SYSTEM_PRINCIPAL, 0);
@@ -373,8 +374,8 @@ static bool WritePrincipalInfo(JSStructuredCloneWriter* aWriter,
   MOZ_ASSERT(aInfo.type() == PrincipalInfo::TContentPrincipalInfo);
   const ContentPrincipalInfo& cInfo = aInfo;
   return JS_WriteUint32Pair(aWriter, SCTAG_DOM_CONTENT_PRINCIPAL, 0) &&
-         WritePrincipalInfo(aWriter, cInfo.attrs(), cInfo.spec(),
-                            cInfo.originNoSuffix(), cInfo.baseDomain());
+         ::WritePrincipalInfo(aWriter, cInfo.attrs(), cInfo.spec(),
+                              cInfo.originNoSuffix(), cInfo.baseDomain());
 }
 
 bool nsJSPrincipals::write(JSContext* aCx, JSStructuredCloneWriter* aWriter) {
diff --git a/caps/nsJSPrincipals.h b/caps/nsJSPrincipals.h
index a7504affbdec..e72323064e33 100644
--- a/caps/nsJSPrincipals.h
+++ b/caps/nsJSPrincipals.h
@@ -9,6 +9,12 @@
 #include "jsapi.h"
 #include "nsIPrincipal.h"
 
+namespace mozilla {
+namespace ipc {
+class PrincipalInfo;
+}  // namespace ipc
+}  // namespace mozilla
+
 class nsJSPrincipals : public nsIPrincipal, public JSPrincipals {
  public:
   /* SpiderMonkey security callbacks. */
@@ -24,6 +30,9 @@ class nsJSPrincipals : public nsIPrincipal, public JSPrincipals {
                                      uint32_t aTag,
                                      JSPrincipals** aOutPrincipals);
 
+  /* For write() implementations of off-main-thread JSPrincipals. */
+  static bool WritePrincipalInfo(JSStructuredCloneWriter* aWriter,
+                                 const mozilla::ipc::PrincipalInfo& aInfo);
   // This class is used on the main thread to specify which principal to use
   // when reading principals data that was set on a DOM worker thread.
   // DOM workers do not use principals from Gecko's point of view, and any
diff --git a/dom/worklet/WorkletPrincipals.cpp b/dom/worklet/WorkletPrincipals.cpp
index 9eef61473e3f..519a4e3089de 100644
--- a/dom/worklet/WorkletPrincipals.cpp
+++ b/dom/worklet/WorkletPrincipals.cpp
@@ -6,7 +6,7 @@
 
 #include "WorkletPrincipals.h"
 
-#include "mozilla/Assertions.h"
+#include "nsJSPrincipals.h"
 
 namespace mozilla {
 namespace dom {
@@ -20,8 +20,11 @@ WorkletPrincipals::~WorkletPrincipals() = default;
 
 bool WorkletPrincipals::write(JSContext* aCx,
                               JSStructuredCloneWriter* aWriter) {
-  MOZ_CRASH("WorkletPrincipals::write not implemented");
-  return false;
+  // This is a serialization of the NullPrincipal corresponding to the worklet
+  // environment settings object for the WorkletGlobalScope.
+  // https://drafts.css-houdini.org/worklets/#set-up-a-worklet-environment-settings-object
+  return nsJSPrincipals::WritePrincipalInfo(aWriter,
+                                            mWorkletImpl->PrincipalInfo());
 }
 
 void WorkletPrincipals::Destroy(JSPrincipals* aPrincipals) {